1. Field of the Invention
This invention relates to the field of bipolar transistor output stages, and particularly to methods of enhancing the output transistors"" breakdown characteristics when the output stage is in a high impedance mode.
2. Description of the Related Art
Many interface circuits require output stages which are capable of operating in a xe2x80x9ctri-statexe2x80x9d or xe2x80x9chigh impedancexe2x80x9d mode, in which the output terminal presents a high impedance to the external circuitry connected to it. This external circuitry is often powered with supply voltages which differ from those used by the interface circuit. As a result, the output stage""s output transistors may need to withstand voltages which swing above and below the interface circuit""s power forms without breaking down.
In a conventional bipolar interface circuit, the output stage consists of one or two bipolar output transistors which are emitter follower-connected to the output terminal. To put the output stage into a high impedance mode, the bases of the output transistors are left floating. When so arranged, the high impedance output voltage swing range is limited to the transistors"" open base breakdown voltages BVeco (emitter reverse-biased compared to the base) and BVceo (collector reverse-biased compared to the base). For example, if an interface circuit is powered with a supply voltage VDD of 5 volts, and its output stage includes an emitter follower-connected NPN output transistor having a BVeco of 5 volts and a BVceo of 20 volts, the allowable output voltage swing is given by:
(VDDxe2x88x92BVceo) to (VDD+BVeco)=xe2x88x9215 volts to +10 volts. If the interface circuit""s output terminal is subjected to voltage swings beyond this range, the tri-state requirement may be violated and the output transistor may be damaged or destroyed.
A bipolar transistor breakdown voltage enhancement circuit is presented which overcomes the limitations noted above. The invention extends the high impedance output voltage swing which can be tolerated at an interface circuit""s output terminal which is driven by an emitter follower-connected bipolar transistor.
The present invention is a circuit which, when an emitter follower-connected bipolar output transistor is to be in a high impedance state, connects the base of the output transistor to a voltage which extends the allowable output voltage swing. When the output transistor is an NPN, the invention ties the transistor""s base to a voltage that is the lower of the interface circuit""s output voltage or ground (or another low impedance voltage path). For a PNP output transistor, the transistor""s base is tied to a voltage which is the higher of the interface circuit""s output voltage or VDD (or another low impedance voltage path). By shorting the base in this way, the output transistor""s breakdown characteristic is enhanced, such that the allowable voltage swing is extended by up to 50% or more.